Polybutylene terephthalate (PBT) resin is a well-known polyalkylene terephthalate that is semi-crystalline and has several desirable properties alone or in blends. Compared to amorphous resins such as acrylonitrile butadiene styrene (ABS), polycarbonate, and polystyrene, a semi-crystalline resin like PBT can show higher solvent resistance, strength, and stiffness due to the presence of crystalline spherulites in the resin. PBT resin is used in many applications in which its solvent resistance, strength, rigidity, and lubricity are needed, most commonly in durable goods that are formed by injection molding. Through its many blended products, PBT can be tailored to a wide variety of applications, including electronic and communications equipment, computers, televisions, kitchen and household appliances, industrial equipment, lighting systems, gardening and agricultural equipment, pumps, medical devices, food handling systems, handles, power and hand tools, bobbins and spindles, and automotive parts in both under-the-hood and exterior applications. Additionally, PBT is widely used to form electrical connectors.
It is generally known that PBT can be made by reacting 1,4-butanediol (BDO) with terephthalic acid (TPA) or dimethyl terephthalate (DMT) in the presence of a transesterification catalyst. U.S. Pat. Nos. 7,129,301; 6,020,393; 4,328,059, and US Patent Pub. 2005/0113534 A1 disclose various catalysts for the polymerization of polyesters.
Commonly used catalysts for the polymerization of PBT include tetraalkyl titanates. Among the various titanates are tetraisopropyl titanate, tetrabutyl titanate, and tetra(2-ethylhexyl) titanate. For example, JP 60147430 discloses a method of producing polyester by esterifying terephthalic acid, adipic acid and 1,4-butanediol in the presence of a titanium compound and a pentavalent phosphorus compound. U.S. Pat. No. 6,303,738 discloses a process for producing copolyester containing adipic acid in the presence of TYZOR® IAM (available from DuPont), which is prepared through the combination of tetraisopropyl titanate (TPT) and a mixture of butyl phosphate and dibutyl phosphate.
The catalyst used to prepare PBT is not typically quenched (deactivated) at the end of the polymerization process. Unfortunately, an active catalyst in the resin composition can sometimes lead to undesirable reactions in subsequent processing of the polyalkylene terephthalate. On exposure to high temperature and humidity, blends and compositions containing the polyalkylene terephthalate can exhibit hydrolytic degradation. Another problem associated with the presence of the active catalyst is transesterification, which can lead to loss of mechanical properties.
Catalyst quenchers such as phosphoric acid can be added to thermoplastic compositions to prevent such transesterification, but they can also promote degradation of polymer chains and contribute to a decrease in polymer molecular weight as well as greater hydrolytic instability. The use of phosphite stabilizers is less satisfactory because of the tendency for phosphites to be unstable to both hydrolysis and oxidation. Although the use of chain extenders can help to counterbalance the effect of the quencher, it is desirable to eliminate the use of either quencher or chain extender additives as a necessity.
Insufficient hydrostability of polyalkylene terephthalate can lead to chain cleavage, the extent of which depends on the exact conditions of exposure to water or humidity. Temperature, time of exposure, and pH are all important. Both acids and bases can catalyze ester hydrolysis. Decomposition of polyalkylene terephthalate can be accelerated in aqueous acid or base, or if the polymer matrix of polyalkylene terephthalate contains free acid or base additives. Since one of the reaction products of polyester hydrolysis is itself a carboxylic acid, the hydrolytic decomposition of PBT is autocatalytic, as depicted in Scheme 1.

Thus, a need remains for new and improved catalysts or processes for the production of polyalkylene terephthalates that are effective in polymerization, but which do not adversely impact the properties of the resulting polyalkylene terephthalate. There is also a need to eliminate the necessity of using either quencher or chain extender additives in preparing polyalkylene terephthalates. There is a further need for polyalkylene terephthalates with improved properties including hydrostability as well as polymer compositions derived therefrom.